Permanent magnet synchronous machines such as motors with non-sinusoidal back-EMF's often produce significant torque ripple. Torque ripple is a harmonic variation in motor output torque that typically occurs as a result of the interaction of back-EMF and stator current harmonics, as well as cogging torque. Torque ripple is undesirable during operation of a motor due to the vibration, noise, and variation in drive torque that it causes.
Solutions to the problem of torque ripple by those in the art typically fall into one of two categories: improving the design of the motor; and controlling the current input to the motor. For controlling the current input, a significant amount of work has been done to mitigate torque ripple by, for example, controlling stator current harmonics. To determine the values of stator currents that are input to the motor, several methods focus on open-loop systems requiring extensive analysis of a particular machine to determine optimal current harmonics, either for torque ripple mitigation, minimization of RMS current amplitudes, or a combination of both. Particular compensation models rely upon estimates of the torque ripple based on complex mathematical models. However, these models may not always accurately predict the amplitude and/or phase of the actual torque ripple, which may vary considerably as a function of load and drive voltage conditions.
To characterize the level of torque ripple or provide model validation, example systems employ in-line torque transducers. However, the transducers require a particular degree of bandwidth, and a relatively large inertia between the torque transducer and the load. In another example of measuring torque ripple, the user measures machine currents during normal operation of a machine, and uses current injection to reproduce currents and a static torque measurement system to measure torque.